Device for grinding or polishing workpieces by means of an abrasive, having a container (2) and with a rotary disk (3) located therein and rotatable relative thereto.
Such devices are known in the form of centrifugal sliding grinding machines, which comprise a two-part container with a shell-shaped, rotatable container lower part forming a bottom and a stationary, cylindrical container upper part.
Such grinding machines are used for the surface machining of grinding articles, e.g. smaller parts and workpieces, which are moved in the container together with the abrasive objects and optionally a liquid process medium. If the lower part is rotated, the workpieces to be treated are moved outwards on the disk until they encounter the inner wall of the container, where they are decelerated. A rotary workpiece movement occurs as a result of the subsequently supplied workpieces and this leads to an intense grinding or polishing action.
It is a disadvantage of such centrifugal sliding grinding machines that the sealing of the annular clearance and the guidance of the faces bounding the same give rise to considerable problems, which can only be overcome with considerable effort and corresponding costs.
There is a danger that the upper and lower parts, particularly the lower part, will be very greatly heated as a result of friction, if parts of the grinding article and/or additionally added abrasive objects during operation enter the gap between the container bottom and the rotary disk. This leads on the one hand to an only relatively short regrind life of the grinding machine and on the other the latter must be frequently switched off during the machining of grinding articles in order to prevent overheating both of the grinding machine and also the grinding or polishing article.
DE 197 28 931 A1 discloses a liquid grinding machine for the liquid working of workpieces. A substantially planar elastic disk is provided, whose radial extension exceeds the radial extension of the surrounding container, so that on inserting the disk in the container its flexible rim or edge is directed upwards and firmly engages on the container wall, the compressive force being reinforced by the centrifugal force during rotation of the disk. The liquid necessary for wet working is introduced into the container below the disk and under the pressure overcoming the aforementioned application forces is forced between the disk rim and container wall into the machining chamber. As a result of considerable technical and constructional effort and expenditure, a penetration of grinding material into the area below the rotary disk is prevented, but in this form is only possible with wet machining.
The problem of the invention is to provide a simply constructed, inexpensive device for polishing and grinding, whilst avoiding the aforementioned disadvantages, which in the case of limited wear susceptibility functions reliably and has a long regrind life.
According to the invention, in the case of a device of the aforementioned type, this problem is solved in that the rotary disk has an upwardly drawn, elastic disk rim and that the disk rim has a finite spacing from the adjacent inner wall of the container.
The finite spacing or gap, unlike in the case of DE 197 28 931 A1, is constructionally determined by a radial minimum dimension of the rotary disk with respect to the radial inside dimensions of the container and consequently also exists in the inoperative state, particularly without the action of any external forces, such as occur in the prior art as a result of the liquid forced through between the disk and the container wall. Thus, the elastic and in particular flexible disk rim is not in contact with the container wall when the device is in the inoperative state when the disk rim is not burdened by introduced material. The invention deliberately takes into account the fact that the abrasive particles can pass between the disk rim and the container wall, as well as between the disk bottom and the container bottom. This is counteracted by the centrifugal force exerted on such particles by the rotation of the rotary disk and this leads to the particles being directed radially outwards below the disk bottom, which is aided by the elastic and preferably flexible underside of the disk bottom. As a result of this and due to the elastic, flexible design of the disk rim, abrasive particles located between the latter and the container wall are returned to the grinding chamber. A dynamic equilibrium of the material flow occurs and there is always a certain mount of abrasive particles, which form an abrasion-reducing lubricant between the disk rim and the container wall.
According to a preferred development, the disk rim tapers in pointed manner towards its free circumferential edge. As a result the disk rim can be given a high flexibility, particularly in its outer, free edge region.
According to a highly preferred development, the disk rim is inclined outwards and has an inclination to the horizontal of less than 90xc2x0, preferably between 30 and 70xc2x0. The (inner) opening or aperture angle of the disk rim to the horizontal is consequently between 150 and 110xc2x0.
The size of the gap or the finite spacing of the disk rim from the container wall is preferably between 2 and 10% of the disk diameter and therefore, for conventional disk diameters of 15 to 40 cm, generally between 0.4 and 2 mm, preferably between 0.7 and 1 mm.
According to other preferred developments, the bottom of the rotary disk substantially has a constant thickness, the disk rim having a smaller thickness than the disk bottom or the outer circumference (towards the raised disk rim) of the disk bottom tapers.
It can be provided that the disk bottom thickness is between approximately 2 and 8% of its diameter and that the rotary disk thickness is between approximately 5 and 10 mm.
The disk is also spaced from the container bottom. In a preferred development, the vertical spacing is between 1 and 2 mm. The gap width can in particular also be variable, so as to adapt the gap to the granular material used. The adjustability of the gap can be made possible by random known means, e.g. by a washer or the like placed between the container bottom and the disk and as a result a shaft passing through the container bottom for the mounting of the flexible disk can be vertically adjustable and fixed at a random height. Alternatively the container can be vertically adjustable with respect to the disk and can be fixed in a desired height. According to a highly preferred development, the size of the gap between the disk rim and the container wall is smaller than the axial spacing from the disk to the container bottom. This ensures that it is only possible for particles to pass below the rotary disk which are much smaller than the spacing of the rotary disk bottom to the container underside.
According to a highly preferred development, in the transition area from the container bottom to the container wall, an increased spacing is formed with respect to the rotary disk and in particular the circumference of the disk bottom and which is in particular formed by a notch in the passage of the container wall towards the container bottom. This ensures that the rotary disk bottom, which is radially relatively firm, does not strike against the lower area of the container wall as a result of thermal expansion. In the container wall notch it is possible for abrasive material to collect and this consequently guides the returned, loose abrasive particles. As a result of its elasticity and the abrasive particles acting as a lubricant there, a radial thermal expansion in the vicinity of the disk rim is not critical.
It has been found that there is no increased heating under friction and in particular no fusing takes place.
Besides the choice of a flexible material, this is also helped by the shape of the circumferential rim, in that the latter is slender in cross-section and pointed from the bottom area of the disk towards the free edge or lip, so that assistance is also provided by the high lip flexibility.
According to a further development of the invention, the rotary disk is made from plastic, particularly polyurethane. In a preferred development, at least the inside of the container and preferably the entire container is made from plastic, preferably polyamide 6 (PA6) or polyamide 66, the Shore hardnesses of the materials preferably being between 50 and 95xc2x0.
It has been found that such polyamides, particularly based on polycaprolactam, constitute an optimum material for the container wall. In the case of standard abrasive materials or abrasives, the said material is not subject to any abrasion and wear. In addition, a fusing with the rotary disk when suitable materials are used for the latter, once again polyurethane, is reliably prevented. As a result of the use of PA6, the self-cleaning effect of such abrasive parts which pass into the gap and immediately are moved out again, is significantly improved compared with a PU-PU pairing.
The invention also relates to a grinding system with a device according to the invention for the polishing and grinding of workpieces and with an abrasive having organic grains, the latter more particularly consisting of natural, organic material, such as walnut or coconut shell, wood, cherry stones, etc. or the abrasive grains consist of synthetic-organic material, particularly plastic. The abrasive is preferably a composite abrasive with a central, organic material grain, which is surrounded by a binder layer containing polishing particles. In a preferred development, the polishing grain composite has a central walnut grain, which is enclosed by a fat or wax layer, in which are incorporated polishing grains, particularly of aluminium oxide. This abrasive has proved to be of an optimum nature when used in a grinding machine with a polyamide 6 casing and in particular also with a polyurethane disk, in order to on the one hand obtain good grinding results and on the other to prevent damage of the grinding article or workpiece and also the container wall and grinding disk, both through the otherwise occurring jamming and blocking of the drive. The abrasive grain size can be between 50 and 500.
Another preferred development is characterized in that below the disk in the bottom of the container is provided a closable outlet.
According to another highly preferred development, a drive axis of the grinding disk passes in liquid-tight manner through the container bottom.
According to other developments, on the container and below the disk is provided a rotary drive for the latter, whereby the axes of the disk and the rotary drive are aligned.
This preferred development makes it possible to design a grinding machine in a simple and inexpensive manner. This is helped by the fact that the disk is connected in bearing-free, non-rotary manner to the driven shaft of the rotary drive. Through this preferred solution there is no need to provide a separate bearing for the driving shaft of the rotary disk in addition to the bearing for the driven shaft of the rotary disk, where the latter may be a driving motor or a geared motor with integrated motor and integrated gear. According to a preferred development, the disk is connected by means of a coupling to the driven shaft of the rotary drive and in particular the coupling has aligned bores. which carry the driven shaft of the rotary drive and a centring pin for the disk. The coupling and the driven shaft are connected in non-rotary manner by a radial clamping bolt. The rotary disk and coupling are interconnected in non-rotary manner by means of screws. There is a dust-tight and/or liquid-tight passage of the disk drive through the container bottom and the coupling is in particular held firmly in the containers by retaining rings, so that an optimum sealing action is obtained.
According to other developments of the invention, the rotary drive is held in a foot or base part and firmly connected thereto by screws and the fastening screws for the rotary drive are oriented parallel to the axis.
The casing is preferably in one piece and in particular the casing and/or container is made from plastic. As a result the grinding machine according to the invention can be manufactured economically and therefore inexpensively. This is helped by the fact that the gear is placed below the disk. In an alternative development, the drive is constructed as a geared motor with an integrated drive and then in particular the motor is vertically oriented with its axis and a driving shaft passes out at the lower end of the motor.
As a result of the construction of the device or grinding unit according to the invention, a machine can be provided with several grinding units at limited cost, so as to e.g. permit the rational machining of heavier and/or shock-sensitive workpieces, which do not allow a common machining of several workpieces in a single container. In fact, a large number of units can be provided, e.g. more than 30. The disks of the individual containers can either be separately driven or the grinding machine has a common drive for all the units. In the latter case the disks of the units in each case located on one shaft can be connected by coupling elements, such as meshing gear wheels, V-belts, etc., to the central drive and the disks of individual containers can preferably be separated uncoupled from the central drive, so that during the grinding of workpieces in certain containers of the grinding machine, the other containers can e.g. be cleaned or emptied and then again filled with workpieces. A grinding machine according to the invention can in particular be used for the dry polishing of jewellery, dental parts, etc.